1. Field of the Invention
The present invention relates to the process for preparing polyamines which are particularly useful in the preparation of polyisocyanates. More specifically, the present invention relates to the reaction of aniline with formaldehyde in the presence of an acid to produce mixtures of polyamines which are particularly useful in preparing )lighter color polyisocyanates.
2. Brief Description of the Prior Art
Generally, it is known in the art to react aniline with formaldehyde in the presence of an acid to produce polyamines, and to use said polyamines to produce polyisocyanates for use in the preparation of foams. Lately, the color of the foams has become one of the requirements for judging the quality of foams. Foams which are otherwise acceptable can be rejected because of dark color.
Impurities in the polyamines which are used in preparing polyisocyanates for use in the preparation of foams have, at least in part, been blamed for the discoloration in foams. Hence, research in this area has focused on the preparation of polyamines which can be used in preparing lighter color polyisocyanates and foams.
U.S. Pat. No. 4,792,624 discloses that polymethylene polyphenyl polyisocyanates of improved color can be obtained from certain polyamines which are prepared by the following process. The process comprises the preparation of the corresponding polymethylene polyphenyl polyamine by condensing aniline and formaldehyde in the presence of an acid catalyst which is characterized by adding a minor proportion of a polyamine mixture comprising di(aminophenyl)methanes and oligomeric polymethylene polyphenyl polyamines, (collectively known as polymeric MDA to an intermediate stage of the condensation reaction where the various intermediately formed aminobenzylamines are present.
U.S. Pat. No. 4,465,639 discloses addition of controlled amounts of water to the reaction mixture produced by phosgenation of a mixture of polymethylene polyphenyl polyamines (and the like polyamines produced by condensation of formaldehyde and aromatic amines) prior to complete removal of excess phosgene gives rise to the corresponding polymethylene polyphenyl polyisocyanates having significantly improved properties such color of the polyisocyanates.
By the present invention, there is provided an alternate means of obtaining lighter color polyisocyanates.